Highway pole structures utilized on federally funded highways projects must meet the breakaway performance criteria when struck by errant vehicles in order to diminish as much as possible serious injury to the occupants of such vehicles resulting from striking such pole structures. Originally, highway poles were wood, steel or concrete, and such poles provided such rigid resistance to impact from vehicles before breaking away that the injury to the occupants was extremely severe or even fatal. Moreover, when such poles did break, not only were the occupants subjected to probable further injury after impact by the broken off pole crushing the top of the vehicle, but a jagged stub was often left extending above ground a substantial distance, presenting an additional hazard in the likelihood of damaging the steering or underside of the vehicle, or of crushing the vehicle and occupants from the front after impact.
The present required breakaway performance criteria are set forth in the American Association of State Highway Transportation Officials Publication entitled "Standard Specifications for Structural Supports for Highway Signs, Luminaries and Traffic Signals 1985", which states that when a standard 1800 pound vehicle (or its equivalent) travelling at 20 MPH (29.3 feet per second) strikes the luminary support at 18 inches above ground level (as specified in National Cooperative Highway Research Program (NCHRP) Report 230, page 41), the speed of the vehicle is not to be reduced after break away more than 15 feet per second, and preferably not more than 10 feet per second. Another requirement is that any remaining structure after breakaway shall not extend more than four inches above the ground level. Previously these requirements were based upon 2250 pound vehicles.
Prior pole structures for meeting the criteria with the heavier vehicles, such as grooved breakaway bolts and anchorbase fittings of frangible material, have not been satisfactory when tested with lighter vehicles weighing a minimum of 1800 pounds.
In attempting to meet these requirements, highway utility poles have been constructed of lightweight materials such as aluminum and fiberglass, but difficulties have been encountered in providing poles strong enough to resist wind and weight loads while meeting the breakaway requirements, and in avoiding leaving jagged stubs extending more than 4 inches above ground after breakaway.